1. Field of the Invention
The present invention relates generally to molding machines and, more particularly, to an injection molding machine that uses a single electric motor to drive both a hydraulic motor for the charging of an accumulator, and a drive means, for example a feed screw and/or a mold closing means.
2. Description of the Related Art
The injection unit of an injection molding machine provides essentially two functions during the course of a normal-cycle of operation; namely, injection and extruder. In a standard reciprocating screw injection molding machine, the extruder function is accomplished when the screw is rotated, gradually moving plastic melt toward the forward end of the screw, thereby creating a pressure or force to move the screw rearward to its pre-injection position as the melt accumulates. When a sufficient amount of material is accumulated (“a shot”), the screw is moved rapidly forward (without rotation) to inject the melt straight into the mold, thus performing the injection function. The processing requirements for injection molding commercially significant plastics materials involve injection pressures of at least 15,000 psi, and frequently up to 30,000 psi.
The injection unit of a molding machine can also be designed as a “two-stage” system where the extruder and injection functions are performed by separate machine elements. In a two-stage injection system, the extruder or plasticizing function is still performed by a feed screw in a heated barrel, but all or part of the plastic melt is diverted into a “melt-accumulator” rather than being conveyed directly to the mold. The melt-accumulator is subsequently operated to perform or, at least, assist in performing the injection function. The advantages of a two-stage injection unit include more uniform plastication of material, reduced wear on the screw and barrel, and the potential for higher injection pressures. The primary disadvantage is higher cost.
Both the injection and extruder functions require an associated drive apparatus in the injection unit. In prior art hydraulic machines, the movement for the injection function is typically performed by a hydraulic cylinder, while the rotation of the feed screw for extruder run is normally accomplished by a hydraulic motor. More recently, electric motors combined with mechanical systems have been used as the direct power source in the injection unit. Some of the prior art electric systems have used separate motors for each function; i.e., one motor for rotating the feed screw and a second motor in combination with a mechanism, such as a ball screw, to convert rotary motion into the linear movement required for injection. Other prior art “hybrid” machines have used an electric motor to rotate the feed screw with the remaining functions of the machine being hydraulically driven, with power provided by an electric motor driving one or more hydraulic motors.
While the “hybrid” machine incorporates some of the advantages of both electric (better control of screw rotation) and hydraulic (lower overall cost) machines, there remains room for improvement. In particular, there is potential for a more economical system since there is excess capacity in the electric motor that rotates the screw. This motor is only used during the portion of the cycle were the thermoplastic material is extruded (plasticated) to build the shot. Since the motor and the associated variable speed drive have a relatively high cost, it is desirable to maximize the utilization of this motor. Furthermore, for the injection molding machines with variable speed motors currently available, the motors are either dedicated to specific axes (as with electromechanical systems), or are applied to standard hydraulic circuits redundantly so that no economy of control is gained by the variable speed motor and drive.
Accordingly, as is typical when new technology is applied to existing products, the effort has been to maximize the execution of the previous injection system technology so as to limit risk and retain product identity. This is especially true in all-electric injection molding machine design where hydraulic motion control has been replaced with electro-mechanical motion control. As a result of this limited design approach, many important advantages of electric variable speed motor drives have not been realized in their application to injection molding.
It is well established that simply replacing hydraulic drive trains with electromechanical drive trains provides significant, measurable improvement in repeatability, stability, and accuracy of the driven device. This is a result of reducing the number of components in the drive train, elimination of inherent variations in the hydraulic fluid as a function of temperature, viscosity changes due to ultimate chemical breakdown of the oil itself, eventual increasing concentration of contaminants, and so forth. However, while simply replacing the hydraulic drive train components with servo-electrical/mechanical components provides desirable performance improvement, the full potential improvement has yet to be realized.
Another consideration is that the floor space occupied by an injection molding machine has become an increasingly important criteria. As the resources once available for facilities are diverted to other assets to increase productivity, the length, width and height of a machine has become increasingly important consideration among competing machine designs.
Besides the need for increased capacity in electric injection units, there is potential for improvement in durability, repeatability, stability, and accuracy of the driven device, as well as a reduction in overall length of the machine, if a way can be found to overcome the obstacles presented by limiting application of electro-mechanical technology to reciprocating screw injection units.
Similarly, there is a need for an improved energy efficient system when operating a closing or clamp unit of an injection molding machine where the two halves are movable towards or away from each other for opening and closing the injection mold. In this arrangement the injection mold must be subjected to a relatively large closing force during the injection cycle. The prior art centers around apparatus that utilizes all hydraulic actuation for both the long stroke portion of opening and closing the mold as well as for applying the clamping force. More recently, electric motors have been used for the long opening and closing stroke and hydraulic pressure is utilized to apply the large clamping force during the injection cycle. The prior art however has yet to provide a compact, energy efficient drive system utilizing both electric motors and hydraulic motors.